The telecommunications network and other similar networks, such as intra-computer interconnects, are increasingly relying upon optical fibers for high-speed, noise-free data channels. However, optical networks generally suffer from their inability to switch an input signal between two or more output channels without the necessity of converting the optical signal to an electrical signal and using more conventional electronic switching. Various types of optical switching have been proposed, such as integrated optical devices (for example, directional couplers), SEED devices, non-linear optics, and acousto-optical converters. However, to date, all of these proposed solutions have required relatively advanced technology and thus have been considered uneconomical to implement in a practical network.
Of the optical switching devices so far proposed, most have severely suffered from their sensitivity to polarization. In one application, these optical switching devices are used at some intermediate point between the source and the destination of the optical path. However, the fiber causes the polarization state of the optical signal it outputs to be indeterminant, and the polarization seems to vary randomly with time. As a result, any polarization-sensitive switching device receiving a signal from such an optical fiber will display a switching characteristic that uncontrollably varies with time. In U.S. Pat. No. 5,002,349, Cheung et al. have proposed a polarization-insensitive acousto-optical switching element. Although this device offers promise, at the present time its fabrication is difficult and expensive, and its eventual usefulness remains uncertain.
Patel has proposed a number of frequency-selective, liquid-crystal devices. In particular, in U.S. Pat. No. 5,111,321 incorporated herein by reference, he has described a drop-add circuit which can selectively switch one frequency component out of an optical channel. Further, in that patent, he has described a polarization-insensitive optical filter in which the input signal has its two polarization states spatially divided by a calcite crystal.
Soref has proposed a liquid-crystal optical switch in "Low-cross talk 2.times.2 optical switch," Optics Letters, vol. 6, 1981, pp. 275-277. His switch depends on both a liquid-crystal device and polarization-splitting prisms. His prisms are considered to be impractical because they require liquid-crystal devices of uniform characteristics over a large area. Soref et al. have proposed another liquid-crystal switch in "Calcite 2.times.2 optical bypass switch controlled by liquid-crystal cells," Optics Letters, vol. 7, 1982, pp. 186-188. This device substitutes calcite crystals for the earlier prisms. Three calcite crystals sandwich two liquid-crystal modulators. Half-wave plates are inserted into one of the polarization-differentiated beams so the unpatterned twisted nematic liquid-crystal modulators act similarly on the two beams. Soref et al. disclose that the separation of the two beams, partially dictated by the half-wave plates, to be 2.7 mm, which is still too large for a rigorously uniform liquid-crystal device. Fujii has proposed a compact version of the Soref system in "Low-Crosstalk 1.times.2 Optical Switch Composed of Twisted Nematic Liquid Crystal Cells," IEEE Photonics Technology Letters, vol. 5, 1993, pp. 206-208. His system includes two assemblies on each end including a bulk polarization beam splitter and a segmented liquid-crystal modulator. A birefringent crystal is placed between the two assemblies. Wagner et al. in "Electrically controlled optical switch for multimode fiber applications," Applied Optics, vol. 19, 1980, pp. 2921-2925 have proposed a polarization-diversity switch using polarization beam splitters and twisted nematic liquid crystals.
None of these prior-art liquid-crystal switches offers a practical rugged optical switch. The desired optical switch has minimum separation between the various beams so that any spatial non-uniformities of the liquid-crystal device in the lateral directions minimally affect its characteristics. The device should be compact. Preferably, it should consist of planar layers so that its layers can be easily bonded into an integral whole. The basic configuration should be easily expandable to a larger switch with little increase in complexity.